2 April 2018

Reluctance Motor : Construction, Working, Advantages, Disadvantages and Applications

Single Phase unexcited Synchronous Motor

  • The characteristic of single phase unexcited synchronous motor in which excitation does not necessary is given below.
        ( 1 ) It operates on single phase supply. 
        ( 2 ) The stator rotating magnetic field rotates at synchronous speed. 
        ( 3 ) It does not require DC Excitation. 
        ( 4 ) It is self starting.

Reluctance Motor

Principle

  • When a magnetic material is placed in the magnetic material, it always aligns in the minimum reluctance path.

Construction

Stator

  • The stator of the reluctance motor is similar to that stator of the single phase induction motor
  • It consists of starting and running winding in the stator slots. 
  • This type of motor is also called as Split phase reluctance motor.

Rotor

  • The rotor of the reluctance motor is of salient or projecting poles. 
  • Let us consider that the rotor of the squirrel cage induction motor consists of 24 copper bars. 
  • If the rotor bar 5, 6, 11, 12,17,18,23 and 24 are cut, it is similar to 4 salient poles.

rotor of the reluctance motor

Working

  • When a single phase supply is given to the stator winding, a rotating magnetic field is produced in the stator winding. 
  • When a salient poles rotor cut this magnetic field, rotor aligns in the minimum reluctance path due to reluctance torque. 
  • The reluctance depends upon air – gap between stator and rotor.
  • Figure A shows 4 pole salient pole rotor in which direction of four high Permeance and four low Permeance is shown.
  • High Permeance means higher magnetic conductivity and higher inductance. Similarly low Permeance means lower magnetic conductivity and lower inductance.
  • The reluctance is inverse of Permeance. Low reluctance means higher inductance and vice versa.
        L α N2 / S
        Where L = Inductance and
                   S = Reluctance of magnetic path
  • Low air – gap means low reluctance and vice versa
        S = L / μ0μra
        Where L = Length of air – gap
                 μ0 = Absolute permeability = 4π × 10 – 7 Henry / meter
                 μr  = Absolute permeability
                  a  = Area
  • There is low reluctance path between stator and salient poles due to small air – gap whereas high reluctance path between stator and inter – polar axis due to large air – gap. 
  • The reluctance motor starts as an induction motor. 
  • When the rotor rotates at its maximum speed, it aligns with the stator synchronous magnetic field due to reluctance torque.  
  • The angle between stator poles and rotor poles of opposite polarity is called as torque angle. 
  • As the torque angle increases, the reluctance torque also increases. 
  • The maximum reluctance torque attains at torque angle of 450
  • The load taken by the reluctance motor is only fraction of the load taken by the three phase inductance motor.

Advantages

  • Low maintenance
  • DC supply not necessary
  • Simple construction
  • Constant speed characteristic

Disadvantages

  • Low efficiency
  • Low power factor
  • Only fraction of load taken as compared to three phase induction motor

Applications

  • Automatic regulator
  • Signaling devices
  • Recording instruments
  • Tele printer
  • Timer circuits
  • Gramophone
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